PHOTOSYNTHETIC INDUCTION OF DUAL PHOSPHATE-UPTAKE KINETICS IN PORPHYRA-UMBILICALIS

The rates of phosphate uptake and photosynthesis were simultaneously d etermined in order to investigate the relationships between phosphate use and photosynthesis. Porphyra umbilicalis (L.) Kutzing exhibited tw o different phosphate uptake kinetics. The first one followed a satura tion model and was observed in light (maximum phosphate uptake rate, V -max = 94 +/- 30 nmol P-1 m(-2) s(-1); semisaturation constant. S-0.5 = 4.0 +/- 3.4 mu M; phosphate compensation point, PCP = 0.3 +/- 0.4 mu M); the second one was linear and worked at high external phosphate c oncentrations in the dark. Inhibition of photosynthesis by removing th e inorganic carbon from the medium produced the same effect as darknes s on phosphate uptake. Successive bicarbonate additions produced incre ments of photosynthesis rate and the recovering of the phosphate uptak e pattern observed in light. The results showed that Porphyra umbilica lis, at the typical phosphate concentrations in its natural habitat, t akes up phosphate in the light through the operation of a photosynthet ically controlled active system.